You can access another network computer to run the extracted installer files from your current OS, to install to another partition and sometimes to overwrite the current partition.
Another method to go diskless is to copy the installer's extracted files to a 3gb Primary partition you create by shrinking your current OS, then run the Setup File from there. This should allow you to overwrite the current OS partition.
Best method is to boot a flash stick installer which has the ISO written to it by Win2Flash or UltraISO. This allows you to use the Custom>Drive options to delete, repartition and format the HD - which can only be done from booting the installer. How to Install Vista on a M200 without a CD Drive using TFTP | RyanTAdams.com